I would like to create a little tool to weld filament pieces together. To control the device, I would like to use an cheap but reliable 3D printer mainboard of which I would only need the heater, temp sensor and hot end fan functionality. As the stepper motor drivers are usually a big cost factor in such boards and I do not need them, I am looking now for a board that comes without them.
Can you recommend a specific board - and maybe even a seller - that could be made to work with a display but without the stepper drivers? Or would I better of to start with an Arduino and a relais shield? Would that require another shield for the temp sensor?

Thanks for the links. If it is that easy, I think I will start with one of the Arduinos that I have laying around at home. The display is not a hard requirement - more like a nice to have. A multi-color LED to show, if the temperature has been reached, and a dip switch for the temperature selection or something like that will do in the beginning.

I liked the idea but since it is not very practical to always turn around and open up my UM2, I wanted to create a similar device using a standalone electronic. And since I still have the old original nozzle of my UM2 laying around unused, I thought that I could use that one together with a replacement heater and ptc to construct a similar device.

Maybe you could manufacture a custom ptfe tube with exactly 2.85mm (with expensive drill bits it should be posible. Then you just heat the ptfe at a 160C and you can forget about cleaning etc? Justan idea

Would that mean that the PTFE would then have to stay on the filament until you unload it? It would work for adding small pieces before starting a print I assume. But for adding a new roll of filament to the end of the roll currently being used by the printer it would be not so good.

I was hoping to get away with just machining off the existing nozzle of the old block and then creating some similar groves in the remaining block like in the design on Youmagine.

Well you could heat the new filament and take out the ptfe, since there will always be a point of exit (the one used to insert the filament into the machine. Ofc that would mean that you actually need to take out the filament (atomic etc). Indeed for that, that wont work. ummm...

I finally managed to convert the ideas that I had collected over (too many) months into an Arduino shield to drive the filament fuser!

The shield has a 12V input, an optional 12V to 5V step-down converter to drive the 5V part without the need of an USB connection and the necessary parts to drive the fan, the heater and measure the temperature of the hotend with the 100K PTC.

Now I need to work on the software. I have a LCD shield with buttons that I want to use as "user interface". Currently the code can already measure and display the temperature. Next step is to develop the PID code that will control the heater based on the temp sensor readings. Once that is done, I need to code a menu to select and maybe store the target temperature(s).

It is surprising how little modification is needed to convert the Adafruit Sous Vide cooker code example to a working filament fuser code

I used the Sous_Viduino code from Adafruit as a starting as it has a menu, PID and storage for set values and uses the RGB backlight version of the monochrome backlight LCD display and button shield that I have. So I only had to remove the backlight related color changing code and replace the OneWire/DS18B20 based temperature measuring code with my existing code for the 100k PTC. Some more smaller changes to use only one digit precision and here it is: Version 1.0 of the filament fuser and the details on how to build the shield!

It heats up fine and seems to keep the temperature fairly okay after reaching the set temperature. Now I can start with some fusing tests...